1. Field of the Invention
The present invention relates to a structural member molded from a synthetic resin wherein the structural member is connected to another member by threadable engagement or press fitting.
Further, the present invention relates to a method of producing a structural member of the foregoing type by employing an injection molding process.
2. Description of the Related Art
As is well known, a variety of products molded from a synthetic resin have meritorious advantages in that such products have a light weight, excellent anti-erosion properties, can be easily colored and decorated and can be mass-produced efficiently and inexpensively. Therefore, products molded of a synthetic resin have been hitherto extensively used.
In addition, not only an ordinary synthetic resin but also engineering plastics are used as industrial materials for motorcars, televisions, refrigerators and other products.
In recent years, research and development work has been conducted for providing by fully utilizing modern blending technology, a new polymerized material and plastic material having excellent properties.
However, as far as industrial machines are concerned, these plastic materials have limited practical application because various properties and a high rigidity are required when they are employed as an industrial material.
For example, in a case where the foregoing kind of plastic materials are employed for pumps, products molded of a plastic material have been heretofore produced using the same plastic material by employing an injection molding process. However, the plastic material has a tensile strength lower than that of an incorporated metallic material by a figure. Additionally, the plastic material has an elastic modulus lower than that of the metallic material by two figures. Further, the strength of the plastic material is readily reduced under the influence of welds and irregular distribution of a filler during a molding operation.
To connect a pump to a pipe line, the pump must include a part which serves to connect a joint or a plug to a main body of the pump. However, in view of the low strength of the plastic material as mentioned above, this part should not be constructed such that a high intensity of stress locally appears when tapered male threads are threadably fitted to the part or a tube is press fitted into the part. Instead, the part should be constructed such that it is sealed in a liquid tight manner by using a sheet packing or an O-ring on the end surface thereof. Alternatively, the part can be constructed such that a metallic insert is used for the part. With the exception of a case where standard threads of which the pitch is dimensioned to be smaller than M8 (which denotes a metric coarse screw thread having a 1.25 mm pitch) are employed for the metallic insert, fabrication of this metallic insert is expensive when it is designed to have a special configuration or is designed to be of a large dimension as in the case of an insert bush, tapered threads for a pipe line or the like. Accordingly, another problem is that the metallic insert is made additionally expensive by requiring to be corrosion resistant.
Consequently, as long as the conventional method of designing and molding products using a plastic material in the above-described manner is employed for industrial machines, application of the products molded of the plastic material cannot be enlarged due to many restrictive conditions derived from the undesirable properties of the plastic material.